Quartz and Chalcedony Part 3
By Barry Taylor
In this final section on the truly amazing mineral that is known as Quartz, I will be looking at the huge diversity of crystal forms that exist, detailing the most common crystal forms found.
The shape of any individual quartz crystal and the form of crystal groups varies enormously;
colloquial names exist for virtually every distinct form of Quartz in conjunction with the location of each discovery. Non technical names exist for the many different forms, for example Spirit Quartz, this particular form of quartz crystal has its origins in the Magaliesberg Mountains, Gauteng Province, near Pretoria in South Africa. The deposit was originally discovered in 1986 in very ancient host rocks reported to be around one billion years old. The origin of the name Spirit Quartz records that local tribes-people who found the crystals would sell them on to finance the purchase of cheap home produced “spirit”, this name has since been adopted by new age followers. An alternative dealers name for this unusual form of quartz is Cactus Quartz, so named because the variously coloured crystals all have a coating of spiky crystals, making them look similar to a cactus.
A related form to the above quartz that exists as clusters, has a mesh of fine delicate Needle Quartz or Bristle Quartz crystals at the base of the crystal clusters this is just a variation on the same two generation spirit quartz theme. There is yet another related form that is called Barnacle Quartz, in this case small clusters of crystals are attached to a larger crystal in a similar way that limpets or Barnacles adhere to rocks on the sea shore, these attachments are also often of a different colour. All these names are just added by mineral dealer's in order to enticing people to purchase yet another unique example. Though there are many crystal forms that look very similar, all are slightly different and therefore unique. There are differences in form that can be recognised as specific to each of the different places of origin for individual crystals and crystal groups.
This type of quartz crystal form has stalctitic like growths upon a core that may originally have been calcite and now formed by replacement of the initial calcite by quartz; this type of quartz growth is found in Brazil. The stalactites when discovered are later sliced in sections to reveal the multy colourful rings seen in the cross sections; these very beautiful
slices have circular bands of various colours with an obvious hollow stalactite central core and these slices are then used in decorative jewellery.
Other strange varieties of quartz crystal have been found and they include Skeleton Quartz, also known as Hopper Quartz, Window Quartz or Elestial Quartz, no wonder there is such confusion. This particular form of quartz consists of doubly terminated crystals without any obvious attachments, many of the crystal faces have developed stepped sunken surfaces that look like the classic hopper form of salt crystals. The staggered growth builds up selectively because of an increased growth rate at the crystal edges in preference to the flat faces. This crystal form is found as what is called a floater form, that is a single crystal held in a soft groundmass of clay minerals. Any crystal growth will often incorporate clay or sand particles within the quartz; one of the classic localities for this form of crystal is Kazakstan.
Amongst the many dealer names to be encountered is the Brazilian name of Jacare quartz, the recognised descriptive name for this form is actually Alligator Quartz, because this form has a stepped crystal growth, arranged in such a way that they look like scales sticking up from an alligators back, this form of crystal originates from Ferrugina, Minas Gerais, Brazil.
This strange form of crystal has reportedly been due to selective tectonic pressure during crystal growth in the Mountain building areas where they are found. These crystals are flattened structures and have a staggered growth pattern of re-healed crystals, these are found in the Swat Valley area of Pakistan. The resulting disrupted growth has in this case produced very strange flattened crystals that are referred to as faden quartz; often these crystals have a distinct milky white stringer in the centre of the wide crystal faces.
This type of quartz structure is found when a lager doubly terminated crystal is sitting on the top of a slender quartz column. This form is called a Sceptre, named after the Ceremonial Mace or Sceptre that can be seen in the House of Commons debating chamber.
There are numerous worldwide locations that produce this unique form including many parts of the USA, as well as Madagascar and Ganesh Himal in Nepal. Sometimes this form
of quartz can also be found with what are called reverse sceptres, that is with a small point sitting on top of a larger crystal and this form is well known from Chihuahua area of Mexico. A variation of this form is called Castle Quartz and this has an unusual form of sceptre, where one double ended crystal is surrounded at one end by castle like turret crystals, these look like castellated fortification structures.
Jack Straw Quartz
This is a crystal group assemblage, which occurs when long slender crystals are formed in an interlocking framework, these have the distinctive appearance of long slender rods that are found in the famous children's game of pick up sticks. One good example comes from the famous mining area of Hunan in China; another example comes from the Arenas area of the USA. There are so many special cases of quartz formation that thinking up new names must be a full time occupation, but they can often be fun to look at and try to figure out why they appear as they do. Avariation on this form is the single elongated doubly terminated crystal, referred to by crystal healers as a quartz wand.
This variety was recently obtainable from Cavnik, Rumania and was found as White greenish or Black crystals, some of the most spectacular versions are encrusted with Dolomite crystals. A recent location for this quartz type is the Shangboa Mine, Hunan, China, here the crystals have the classic bulbous form and are covered with fine parallel crystal attachments. The small crystals grow in parallel to the larger quartz faces and the whole form is a globular shaped mass. tapered at both ends. This form of quartz crystal is also referred to as Sprouting Quartz, more specific details can be found using the different search names on various computer websites, have fun.
This beautiful form usually appears as clear doubly terminated crystals with a diamond shape and they come from Herkimer, Washington State, USA, they look like diamonds scattered on the very hard groundmass. often well formed crystals are loose or attached at one end point only hence the name. A very good more detailed article on this location has recently been published in Rock n gem. Similar looking doubly terminated crystals are also found in Tibet as well as in nearby Afghanistan, sometimes crystals have inclusions of a petroleum like fluid, they can often be found at most of the large gem fairs.
Black or White tipped Quartz
An even more unusual variety of quartz, found in Brazil and Russia, comprises groups of quartz crystals that have differently coloured crystal tips, with a visibly different structure that is similar to the sceptres. The Russian version has grey coloured opaque columns capped with a secondary overgrowth of white quartz at the tip. The black tipped version from Brazil however must have formed in a different way, possibly due to a separate pulse of activity which affected the structure of the quartz crystals as they grew. The result is a smoky quartz column with a different dark black cap. I feel that this could be due to a change in the initial fluids of formation, altering structurally part way through the crystals growth. I assume that then natural irradiation has then picked out the different crystal structures, possibly of left or right handedness in the atomic structure, the natural irradiation that takes place alters the colour selectively.
This is the translucent non crystalline variety of silica, which is halfway between quartz and Chalcedony, found as globular masses with no discernable structure, referred to as amorphous as it has no atomic structure and is therefore non-crystalline. This type of silica rich material is closely related to the other non crystalline variety known as Opal; this is also found in colloidal masses where water is incorporated in the atomic structure, giving rise to the characteristic play of colours famous in Australia and also in parts of the USA, Mexico and the Middle East.
This form of silica rich material is formed as a low temperature version of the Quartz-Silica mineral group, this has a microcrystalline form that has many structural varieties based on both chemistry and form, some examples of these follow.
Chalcedony as Agate
This type is usually found in globular masses of silica that have been eroded from the host rock, various inclusions within this chalcedony are found, distinct masses have a reniform or stalctitic structure and there is a constant crossover between Quartz and Chalcedony.
Commonly found in Geodes from Morocco, this type of Silica deposit can be seen in more detail in a previous article which readers should see in Rock n gem volume 54 winter 2011.
Chalcedony is the mineral that forms the bands within agates, this is as a micro-crystalline mineral assemblage with long and short fibres of Chalcedony which have colour enhancement. Some Agate geodes can exhibit a later stage of secondary silica crystallization, grown on the hollows remaining within a visually attractive hollow geode with crystalline centres. Agates of exquisite beauty are often to be seen in gem shows and shops, often cut and polished as thin slices, usually having a distinctive banded appearance that is widely used in decorative Jewellery or as strands of beads.
Is a beautiful bright Apple Green Chalcedony that is coloured by traces of Nickel and exists in massive form.
Chrysophrase originates through the deep weathering of Ultrabasic volcanic rocks such as Nickeliferous Serpentinites and other ophiolitic material, most of this material comes from Queensland in Australia, Germany, Russia, USA and the larges source of Szkary, Lower Silesia, Poland.
These are composed of chalcedony (not to be confused with the crystalline quartz Amethyst roses), Chalcedony Roses seem to occur together with the more complete agate assemblages in related hollow vesicles within certain devitrified lavas. It is not fully understood what the exact mode of formation is, however I have seen evidence that these can occur on the inside of hollow lava vesicle, possibly from where a tube of escape is about to form, perhaps this leads to an incomplete stage in the geodes formation.
other articles in this series:
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